The present invention relates to a hybrid structure for a flexible pipe constituting a riser in off-shore oil production and designed for the transportation of effluent under pressure. The structure according to the present invention is particularly well suited to flexible pipes of the riser type, that is to say to flexible pipes paid out from a surface installation such as a platform and which are connected to the underwater installations and the greater part of which do not rest on the seabed. Such flexible pipes operate in a dynamic situation unlike the flexible pipes which are laid on the seabed and known as flow lines, which work essentially in a static situation.
The flexible pipes or risers which connect the surface installations to the seabed equipment such as a well head or manifold have varying configurations such as, for example, those represented in "Recommended Practice for Flexible Pipe" API 17B, Jan. 6, 1988 edition.
Flexible pipes designed to be used in fairly shallow or moderately shallow water (typically between 100 and 800 m) have structures which can vary widely, depending on the conditions of use.
The flexible pipes most widely used in oil production are generally of the unbonded type, in which the various successive and distinct layers have a certain freedom to move with respect to each other, and they comprise, from the inside outwards, a carcass consisting, for example, of an interlocked metal strip which serves to prevent the pipe from being crushed under the external pressure, an internal sealing pressure sheath made of polymer, a pressure vault consisting of at least one interlocked shaped wire wound into a short-pitch spiral at almost 90.degree., so-called tensile armor whose lay angle, measured along the longitudinal axis of the pipe, is less than 55.degree. and an external sealing sheath made of polymer. Such a flexible pipe whose innermost element consists of a carcass is called rough-bore.
When a flexible pipe comprises, from the inside outwards, an internal sealing sheath, a pressure vault consisting of interlocked shaped wires wound with a short pitch and intended to withstand the hoop stresses caused by the flow of effluent through the flexible pipe, an anti-collapse sheath, one or more tension and pressure armors wound around the anti-collapse sheath and an external sealing sheath made of polymer, such a flexible pipe because the innermost element is a smooth-walled leakproof sheath is called a smooth-bore.
The elements which make up these various structures are defined in documents API 17B and 17J of the document recalled above.
In an alternative form, the flexible pipe has no pressure vault and the layers of reinforcement are spiral-wound with reverse pitch at lay angles of close to 55.degree.. In this case, the internal and external pressures and the tensile forces are exerted or supported by these layers of reinforcement; such a flexible pipe is said to be balanced.
Examples of structures of flexible pipes are described, for example, in FR-A-2,619,193 and FR-A-2,557,254.
Proposed in FR-A-2,619,193 is a flexible pipe whose dimensional variations, particularly in the axial direction, can be controlled in such a way that dimensional stability or controlled shortening can be achieved when the internal pressure is raised.
In FR-A-2,557,254, the flexible pipe is designed not to exhibit an appreciable variation in length when subjected to an internal pressure with a "direct bottom effect" which is induced by the pressure within the flexible pipe.
Described in DE-A-34,40,459 is a flexible pipe which, in particular, comprises a sealed internal assembly and layers of tensile armor. However, there is just one group of tensile armor with a sealing sheath arranged around a winding which could constitute a pressure vault, as is described, in particular, in the API documents 17B and 17J recalled above.
Described in U.S. Pat. No. 4,403,631 is a flexible pipe comprising, from the inside outwards, an internal sealing sheath, a pressure vault consisting of one or more windings of a wire of appropriate cross section, an anti-collapse sheath, several layers of tensile armor and an external sealing sheath. In one embodiment, the lay angle of the winding of the pressure vault being a short pitch angle, for example between 60 and 85.degree., whereas the lay angle of the tensile armor is between 0 and 20.degree.. In another embodiment, the lay angle of the tensile armor is between 75 and 90.degree..
In this last document, the sheath arranged between the pressure vault and the first layer of tensile armor is, as is usually the case in the type of pipe described, an anti-collapse sheath. It cannot in any way be seen as being an intermediate sheath arranged between groups of tensile armor.
In the flexible pipes currently available, the reinforcements are designed to withstand all the loadings they experience during manufacture, transportation, laying, service and recovery.
When the pipes are intended to transport gas alone, or gas associated with liquids, particularly liquid hydrocarbons or two-phase mixtures, it has been proposed that, in order to evacuate the gas which necessarily diffuses through the various elements that make up the flexible pipe, the external coating be made relatively permeable in the emerged part of the flexible pipe, either by the use of holes or by choosing a material which has higher permeability to the gas than does the material of the internal tube. It is also known practice for areas of weakness to be produced in the external covering, particularly grooves or holes which are not open but form places where bursting can occur in the event of overpressure, thus offering a preferred passage through which the gas can escape (bursting discs). In patent FR-A-2,553,859, it is described that a leakage flow be achieved using a recess made in an end fitting with which the pipe is equipped and which communicates with the annular space outside the group of reinforcements armor.
EP 0,341,144 describes a flexible pipe in which there is formed a duct which places the external annular region in communication with the outside environment, the said duct opening, on the one hand, between the external sealing sheath and the uppermost reinforcing armor layer and, on the other hand, into a valve which works on differential pressure.
Irrespective of the type of flexible pipe used to form a riser between seabed equipment and surface equipment, the said flexible pipe progresses through the water in a controlled way so that, up to where it connects with the seabed equipment, hydrostatic equilibrium is obtained within the said flexible pipe.
After the flexible pipe has been connected to the seabed equipment, the flexible pipe is subjected to external loadings which may be classified into two main categories.
The first category essentially consists of tensile stresses which develop, on the one hand, at the connection between the top end of the flexible pipe and the surface equipment and, on the other hand, before and after the buoyancy means that are arranged along the said pipe.
The second category consists of the stresses developed while the flexible pipe is in use, that is to say while effluent is being transported or while the flexible pipe is empty, which corresponds to a shutdown of the flow of effluent (pipe empty). In both instances there is an internal pressure P.sub.int exerted inside the flexible pipe and an external pressure P.sub.ext exerted outside the pipe.
When the difference .DELTA.P=(P.sub.int -P.sub.ext) is positive, the stresses induced in the flexible pipe are radial and longitudinal, the two stresses being considered as positive because they are directed towards the outside of the pipe. The radial stress leads to a radial expansion, while the longitudinal stress leads to a lengthening of the flexible pipe.
When the difference .DELTA.P=(P.sub.int -P.sub.ext) is negative, the stresses are considered as being negative because they are directed towards the inside of the pipe. The radial stress leads to a compression and the longitudinal stress leads to a shortening of the said flexible pipe.
The longitudinal stresses in both instances where the difference .DELTA.P is positive or negative are tensile stresses.
The end cap effect T induced in a flexible pipe depends, among other things, on the pressure difference .DELTA.P=(P.sub.int -P.sub.ext) and on the internal and external radii of the said pipe.
When T is positive, the end cap effect is said to be a direct end cap effect.
When T is negative, the end cap effect is said to be a reverse end cap effect.
Down to a certain depth, the reverse end cap effect has little damaging effect on the flexible pipe, and this is true regardless of the negative value of .DELTA.P.
Beyond a certain depth, the reverse end cap effect may have severe consequences which may lead to damage to the flexible pipe. Thus, that part of the flexible pipe that is situated at a depth of, for example, between 1250 m and 3000 m may be subjected to the reverse end cap effect when the difference .DELTA.P is highly negative, it being possible for the said end cap effect locally to induce a plastic deformation of the reinforcement filaments which may lead to irreversible damage to the flexible pipe.
Furthermore, when the external sheath bursts for whatever reason, the pressure in the annulus increases and becomes equal to the external pressure exerted on the flexible pipe. At a depth of 1500 m, the external pressure exerted on the submerged part of the flexible pipe at this depth is equal to about 150 bar and the reinforcements experiencing this external pressure tend to twist.